Conductive polymeric nanocomposite materials

ABSTRACT

A method of reinforcing a polymeric material with carbon nanofibers is provided in which carbon nanofibers are combined with a polymer and a solvent for the polymer to form a substantially homogeneous mixture, followed by removal of the solvent by evaporation or coagulation. The resulting conductive polymeric nanocomposite material exhibits high electrical and thermal conductivity, enhanced mechanical strength, abrasion resistance, and dimensional stability.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a division of U.S. patent application Ser.No. 09/932,169 filed Aug. 17, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to a conductive polymericnanocomposite material incorporating uniformly dispersed vapor-growncarbon nanofibers, and to a method of forming such a nanocompositematerial.

[0003] Vapor-grown carbon nanofibers are a unique form of carbonproduced by a variation of a vapor-phase catalytic method in which acarbon-containing feedstock is pyrolyzed in the presence of small metalcatalyst particles. The resulting nanofibers typically have an outerdiameter of 60 to 200 nm, a hollow core of 30-90 nm, and a length on theorder of 50 to 100 microns.

[0004] The use of vapor-grown carbon nanofibers has been proposed forproviding improved mechanical, electronic and thermal transportproperties to polymers. For example, vapor-grown carbon nanofibers havebeen dispersed in polymer matrices by a polymer melt blending method inwhich the dispersants in the polymer matrix are mechanically shearedapart. See, for example, U.S. Pat. No. 5,643,502. However, as mostpolymers are incompatible with carbon nanofibers, it is difficult toachieve uniform dispersion of the carbon nanofibers in the polymermatrix. In addition, the high shear mechanical blending can result inthe breakage of the carbon nanofibers.

[0005] Accordingly, there is still a need in the art for an improvedmethod of reinforcing a polymeric material with carbon nanofibers toproduce a composite which has maximum attainable improvement in variousmechanical, electrical, and thermal properties.

SUMMARY OF THE INVENTION

[0006] The present invention meets that need by providing a method foruniformly dispersing vapor-grown carbon nanofibers into polymer matriceswhich enhances their mechanical strength, dimensional stability,abrasion resistance, and electrical and thermal conductivity. Theuniform dispersion of carbon nanofibers in a polymer matrix is achievedby dissolving the polymer in a solvent with the nanofibers, followed byevaporation or coagulation of the solvent.

[0007] According to one aspect of the present invention, a method offorming a conductive polymeric nanocomposite material incorporatingcarbon nanofibers is provided comprising providing vapor grownnanofibers, combining the nanofibers with a solvent to form a solutionmixture, and adding a polymer to the solution mixture to form asubstantially homogeneous mixture. The solvent is then removed from themixture, preferably by evaporation or coagulation.

[0008] In an alternative embodiment of the invention, the method maycomprise combining the carbon nanofibers, polymer, and solvent to form asubstantially homogeneous mixture, followed by removal of the solvent.

[0009] The polymer used in the present invention is preferably selectedfrom the group consisting of polyurethanes, polyimides, epoxy resins,silicone polymers, and aromatic-heterocyclic rigid-rod and ladderpolymers. The solvent is preferably selected from the group consistingof dimethyl sulfoxide, tetrahydrofuran, acetone, methanesulfonic acid,polyphosphoric acid and N,N-dimethyl acetamide. Preferably, both thepolymer and the solvent for the polymer are polar.

[0010] The carbon nanofibers used in the present invention may compriseas-grown fibers, pyrolytically stripped fibers, or heat treated fibers.

[0011] The method of the present invention results in a conductivepolymeric nanocomposite material having a conductivity which may betailored, depending on the desired application, from less than 0.001S/cm to greater than 20 S/cm. Where the conductive polymericnanocomposite material is incorporated with heat-treated carbonnanofibers, the nanocomposite material may have an electricalconductivity greater than 20 S/cm, while materials incorporated with lowconcentrations of as-grown or pyrolytically stripped carbon nanofibersmay be tailored to have an electrical conductivity smaller than about10⁻⁶ S/cm.

[0012] This conductive polymeric nanocomposite material formed by thepresent invention preferably has an electronic conducting percolationthreshold of less than 1% by volume of the carbon nanofibers.

[0013] The polymeric nanocomposite materials formed by the method of thepresent invention may be used to form conductive paints, coatings,caulks, sealants, adhesives, fibers, thin films, thick sheets, tubes,and large structural components. The carbon nanofibers in the resultingnanocomposite materials may be used to confer the desired mechanicalstrength, stiffness, dimensional stability, thermal conductivity, andtribological properties (i.e., reduced surface friction) in suchproducts.

[0014] The nanocomposite materials may be used in a wide variety ofcommercial applications including space, aerospace, electronic,automotive, and chemical industries. The nanocomposite materials mayalso be used in electromagnetic interference shielding, electromagneticpulse applications, electrical signal transfer, electrostatic paintingof panels, electrostatic discharge and electro-optical devices such asphotovoltaic cells.

[0015] Accordingly, it is a feature of the present invention to providea method of forming a conductive polymeric nanocomposite material whichresults in uniform dispersion of carbon nanofibers in a polymericmatrix. Other features and advantages of the invention will be apparentfrom the following description and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] We have found that the polymer nanocomposite material produced bythe method of the present invention is two to three orders of magnitudemore conductive than that produced by a polymer melt blending methodwhen the same amount of carbon nanofibers is used. We have also foundthat the resulting polymer nanocomposite material has a very lowelectronic conducting percolation threshold of less than 1% by volume ofthe carbon nanofibers, which is indicative of an extremely large aspectratio of the carbon nanofibers. This also indicates that the method ofthe present invention is more effective in uniformly dispersing thecarbon nanofibers in the polymer matrices and preserving a large aspectratio of length to diameter of the nanofibers than prior polymer meltblending methods. It is important to maintain the large aspect ratio ofthe carbon nanofibers to confer maximum attainable reinforcement,especially for applications such as the use of elastomeric polymers forforming gaskets or seal structures.

[0017] The method of the present invention achieves uniform dispersionof vapor-grown carbon nanofibers in polymer matrices by dissolving thepolymer in a solvent with the carbon nanofibers. While carbon nanofibersalone do not disperse well in organic solvents, we have found that theydisperse very well in the presence of a polymer. Accordingly, the carbonnanofibers are combined with the polymer and solvent, followed byevaporation or coagulation of the solvent to form the conductivepolymeric nanocomposite material. After the solvent is removed, thepolymer nanocomposite material can be further processed into variousshapes by conventional extrusion and molding techniques without losingits conductivity.

[0018] The method of the present invention provides an advantage overprior melt-blending processes in that a low-temperature solution processis used to disperse the carbon nanofibers. The method does not requirehigh shear mixing of the polymer melt at elevated temperatures, whichtypically degrades the aspect ratio of the carbon nanofibers and leadsto inferior reinforcement.

[0019] In addition, the mechanical and thermal transport properties ofthe resulting polymer nanocomposite material may be tailored by usingdifferent types and amounts of the carbon nanofibers. For example, inEMI shielding applications, the resulting conductivity should be greaterthan 1 S/cm. For electrostatic painting of panels, the desiredconductivity is about 10⁻⁴ to 10⁻⁶ S/cm, and for electrostatic dischargeapplications, about 10⁻⁸ to 10⁻¹⁰ S/cm.

[0020] Preferred polymers for use in the present invention include polarpolymers; however, it should be appreciated that any polymer may be usedin the present invention as long as it is soluble in a solvent.Preferred polymers include polyurethanes, polyimides, epoxy resins,silicone polymers, and aromatic-heterocyclic rigid-rod and ladderpolymers. Preferred polyurethanes include thermoplastic polyurethanes. Apreferred ladder polymer for use in the present invention ispoly(benzimidazobenzophenanthroline) (BBL). The polymer is preferablypresent in a concentration of at least 10 wt %; however, it should beappreciated that the concentration of the polymer may vary depending onthe desired properties and applications, such as coatings, of theresulting composite material.

[0021] Preferred solvents for use in the present invention are polarsolvents and include dimethyl sulfoxide, acetone, tetrahydrofuran,N,N-dimethyl acetamide, methanesulfonic acid, and polyphosphoric acid.

[0022] The carbon nanofibers used in the present invention arepreferably prepared in accordance with U.S. Pat. No. 6,156,256,incorporated herein by reference. Several types of carbon nanofibers maybe used in the present invention including as-grown fibers,pyrolytically stripped fibers, and heat treated fibers. By heat treatedfibers, it is meant that the nanofibers are fully graphitized by slowlyheating the fibers to 3,000° C. and then slowly cooling the fibers. Ithas been found that when carbon nanofibers are heat treated, the orderof the surface chemical-vapor-deposited carbon is highly increased,resulting in substantially improved mechanical, electronic, and thermaltransport properties. Heat-treated carbon nanofibers have an electricalconductivity close to that of highly ordered pyrolytic graphite and athermal conductivity which is five times as high as that of copper. Heattreated fibers are preferred in applications where high conductivity andlow carbon nanofiber loading are desired.

[0023] The method of the present invention is preferably carried out bymixing the carbon nanofibers and the desired polymer in a solvent,preferably in a closed container. While the carbon nanofibers arepreferably dispersed in the solvent prior to addition of the polymer,the nanofibers, polymer and solvent may also be combined at the sametime. The resulting nanocomposite material may be further processedaccording to the desired application. For example, the material may beformed into a thin film which is cast from the solution mixture byevaporating the solvent at a temperature which is below the boilingpoint of the solvent. Alternatively, the solvent may be removed bycoagulation in which the solution mixture is formed into a film or fiberand then immersed in a nonsolvent, such as water, to coagulate the film.The solution mixture may also be formed into thin films by spin coatingand dip coating methods. The solution mixture may also be formed intolarge components such as thick sheets or panels by spraying ordeposition, or by extruding or molding the dried composite material.

[0024] In order that the invention may be more readily understood,reference is made to the following examples which are intended toillustrate the invention, but not limit the scope thereof.

EXAMPLE 1

[0025] A solution mixture was prepared by mixing 0.2 grams of anas-grown carbon nanofiber and 1 gram of a thermoplastic polyurethane in10 grams of dimethyl sulfoxide in a closed glass container. The mixturewas agitated with a magnetic stir bar. A thin film was cast from thesolution mixture by evaporating the solvent at a temperature of about80° C. on a hot plate. The resulting polymer nanocomposite film wasfurther dried in a vacuum oven at 80° C. under reduced pressure. Thefilm had a concentration of 16.7% by weight and 11.7% by volume of thecarbon nanofibers and exhibited an electrical conductivity of 0.25 S/cm.

EXAMPLE 2

[0026] A solution mixture was prepared by mixing 0.2 grams of aheat-treated carbon nanofiber and 1 gram of a thermoplastic polyurethanein 10 grams of dimethyl sulfoxide in a closed glass container. Themixture was then agitated with a magnetic stir bar. A thin film was castfrom the solution mixture by evaporating the solvent at a temperature ofabout 80° C. on a hot plate. The polymer nanocomposite film was furtherdried in a vacuum oven at 80° C. under reduced pressure. The film had aconcentration of 16.7% by weight and 10.2% by volume of the carbonnanofibers and exhibited an electrical conductivity of 5.5 S/cm. Thefilm retained a conductivity of 1.3 S/cm at 100% elongation (stretchedto twice its original length).

EXAMPLE 3

[0027] A series of solution mixtures of a thermoplastic polyurethane anda heat-treated carbon nanofiber were prepared in tetrahydrofuran (THF)using the method described in Examples 1 and 2. Thin films were castfrom the solution mixtures by evaporating the solvent at roomtemperature. The conductivity values of the resulting films are shown inTable 1 below. TABLE 1 Conductivity of polyurethane nanocomposite filmsin relation to the concentration of heat-treated carbon nanofibers.Polyurethane Conductivity Nanofiber (g) (g) THF (g) Wt % Vol. % (S/cm)0.020 1.00 10.0 1.96% 1.12% 0.0038 0.040 1.00 10.0 3.85% 2.22% 0.100.080 1.00 10.0 7.41% 4.34% 0.54 0.120 1.00 10.0 10.7% 6.37% 1.14 0.1601.00 10.0 13.8% 8.31% 3.93 0.200 1.00 10.0 16.7% 10.2% 4.69 0.240 1.0010.0 19.4% 12.0% 8.70 0.280 1.00 10.0 21.9% 13.7% 16.6 0.320 1.00 10.024.2% 15.4% 20.8

COMPARATIVE EXAMPLE 4

[0028] A nanocomposite material was prepared by a melt blending methodin which 20 grams of heat-treated carbon nanofibers and 100 grams of athermoplastic polyurethane were mixed in a Haake Rheomixer at 150° C.for 2 hours. The resulting composite material was pressed into a thinfilm with heat. The film had a concentration of 16.7% by weight and10.2% by volume of the carbon nanofibers and exhibited an electricalconductivity from 0.0052 to 0.0098 S/cm. As can be seen, theseconductivity values are two to three orders of magnitude lower than thatof the nanocomposite film containing the same concentration of thecarbon nanofibers prepared by Examples 1-3.

EXAMPLE 5

[0029] A nanocomposite material of polyimide/amic acid and heat-treatedcarbon nanofibers was prepared from a solution mixture in N,N-dimethylacetamide (DMAC). The solution mixture was prepared by mixing 0.202 g ofheat-treated carbon nanofibers and 1 g of polyimide/amic acid in 10.0 gof DMAC in a closed glass container. The mixture was then agitated witha magnetic stir bar. A thin film was prepared from the solution mixtureby evaporating the solvent at 60° C. The film was further dried in avacuum oven at 200° C for 2 hours. The film had a concentration of 16.8%by weight of the carbon nanofibers and exhibited an electricalconductivity from 1.7 to 2.8 S/cm.

EXAMPLE 6

[0030] A solution mixture of 0.302 g of heat-treated carbon nanofibersand 1.0 g of polyimide/amic acid was prepared in 10 g of DMAC using themethod described in Example 5. The solution mixture was cast into a filmby evaporating the solvent at 60° C. The film was further dried in avacuum oven at 200° C. for 2 hours. The film had a concentration of23.2% by weight of the carbon nanofibers and exhibited an electricalconductivity from 5.1 to 7.7 S/cm.

EXAMPLE 7

[0031] A solution mixture was prepared by mixing 0.02 g of heat-treatedcarbon nanofibers and 0.1 g poly(benzimidazobenzophenanthroline) (BBL)in 20 g of methanesulfonic acid in a closed glass container. Thesolution mixture was agitated with a magnetic stir bar and thendoctor-bladed into a film on a glass slide and immersed in water tocoagulate the nanocomposite film. The film was then air-dried at roomtemperature. The film had a concentration of 16.7% by weight of thecarbon nanofiber and an electrical conductivity of 1.0 S/cm.

EXAMPLE 8

[0032] 11 g of an epoxy resin was mixed with 1 g of heat-treated carbonnanofibers which had been presoaked in 46 g of tetrahydrofuran (THF) toform a uniform mixture by mechanical agitation. 2.86 g of a curing agentwas then added to the mixture. The THF was removed by evaporation andthe composite material was cured at 150° F. for one hour and at 250° F.for another hour. The resulting thermosetting composite materialcontained 6.77% by weight of the carbon nanofibers and exhibited anelectrical conductivity of 0.5 S/cm.

[0033] It will be obvious to those skilled in the art that variouschanges may be made without departing from the scope of the inventionwhich is not to be considered limited to what is described in thespecification.

What is claimed is:
 1. A conductive polymeric nanocomposite materialhaving vapor grown carbon nanofibers incorporated therein, saidnanocomposite material formed by providing vapor grown carbonnanofibers, combining said nanofibers with a solvent to form a solutionmixture, adding a polymer to said solution mixture to form asubstantially homogeneous solution mixture, and removing said solventfrom said substantially homogeneous solution mixture.
 2. The conductivepolymeric nanocomposite material of claim 1 wherein said vapor growncarbon nanofibers are selected from the group consisting of as-grownfibers, pyrolytically stripped fibers, and heat treated fibers.
 3. Theconductive polymeric nanocomposite material of claim 1 comprising afilm.
 4. A conductive polymeric nanocomposite material incorporatingvapor grown carbon nanofibers therein formed by providing vapor grownnanofibers; providing a polymer; combining said nanofibers and saidpolymer with a solvent to form a substantially homogeneous mixture; andremoving said solvent from said mixture.
 5. A conductive polymericnanocomposite material having heat-treated vapor grown carbon nanofibersincorporated therein, said nanocomposite material having an electricalconductivity in the range of about 10⁻⁶ to greater than 20 S/cm.
 6. Aconductive polymeric nanocomposite material having vapor grown carbonnanofibers incorporated therein, said nanocomposite material having anelectronic conducting percolation threshold of less than 1% by volume ofsaid carbon nanofibers.